Mr Johnson did publish an open letter outlining the Foreign Office's position in August in which he said "Mr Tsege's case remains a high priority for the British Government", adding "we take his welfare very seriously".

Image:Campaigners have called for the release of the Briton

Maya Foa, from the human rights group Reprieve, has described Mr Tsege's treatment as "appalling".

She told Sky News: "It's yet another obfuscation by the Foreign Secretary and instead of really looking at this case for what it is, an egregious series of unlawful acts perpetrated against a British national, Boris Johnson is saying he can't answer questions on the case.

"Why? Because he doesn't have the answers because they're not doing the right thing."

Mr Tsege, who became a British citizen after fleeing Ethiopia in 1979, served as a member of the Ethiopian opposition group Ginbot 7.

Ababi Demissie, a minister for public diplomacy based at the Ethiopian Embassy in London, told Sky News the Briton was seized under a treaty with the Yemen government.

Video:Grieve: 'Mr Tsege is a British National'

He said: "Of course he is a terrorist and many people know that.

"We rendered him while travelling to Eritrea. Why was Andargachew flying to Eritrea?

"He was going not for holidays, not to visit his family. He was going to train other fighters to create more chaos and attacks on Ethiopia."